This invention relates to battery backup and standby systems having from one to a large number of jars and, more particularly, to the measurement of jar parameters for use in determining the status of the jar(s).
For lead-acid batteries, it is known in the art that some of the primary measures of battery jar health are the electrolyte resistance, the charge transfer resistance and the double layer capacitance. It is important that the electrolyte solution have the proper acid concentration, which is related to the amount of charge in the jar, and this can be determined by examining the resistance of the electrolyte, which should lie within a defined range. However, the actual electrolyte resistance cannot be measured by itself, since it is only one component (albeit the major one) of the overall ohmic resistance of the jar, which also includes the resistance of the jar grids, terminals, and interconnections or straps. But since the electrolyte resistance is the largest part of the overall ohmic resistance in a properly maintained corrosion-free installation, examining the overall ohmic resistance can provide an indication of the specific gravity of the electrolyte solution. The charge transfer resistance is the resistance between the battery plate and the electrolyte solution, and is related to the condition of the interface between the plate and the electrolyte. Ideally, the charge transfer resistance is zero, but it is considered acceptable if it is below some predetermined value. The plate/electrolyte interface can best be described as two rigid layers of ions which form what is referred to as a "Helmholtz double layer". With increasing plate sulphation and/or grid corrosion, there is less surface area for the inner layer of ions to adsorb to the plate and consequently fewer ions are present in the outer layer closest to the electrolyte. The resultant capacitance (also known as the "double layer capacitance") provides a measure of the plate surface area which is free of sulphation and/or corrosion, and measuring the decrease in capacitance over time provides an indication of the rate of the electrochemical deterioration of the jar. With increasing plate sulphation and/or corrosion, there is a corresponding increase in the charge transfer resistance as well.
It would therefore be desirable to be able to measure the ohmic resistance, the charge transfer resistance and the double layer capacitance of a battery jar, and to be able to do so while the battery jars are on-line in the backup or standby system.